There is a variety of known processes for producing hydrogen. Some examples include the following: (1) steam reforming of natural gas or naphtha (2) catalytic reforming of hydrocarbons, e.g. gasoline and fuel oil, and (3) partial oxidation of coal, heavy oils, or natural gas. Of these processes, steam reforming of natural gas is probably the most widely used process for hydrogen production.
FIG. 1 shows a typical process for producing hydrogen. The process involves reacting methane 110 (e.g., from natural gas) with steam 120 in a steam methane reforming (“SMR”) unit to produce primarily hydrogen and carbon monoxide (sometimes called synthesis gas). The steam reforming reaction proceeds as follow:CH4+H2O=>CO+3H2 
This reaction usually takes place in the reforming unit 130 at high temperature and high pressure. The reaction is equilibrium limited and is highly endothermic. The heat for the reaction is provided by a furnace 137. Fuel 140 for the furnace 137 may be supplied from the methane feedstock 115. The hot exhaust gas 145 from the furnace 137 may be used to preheat feed 138, and generate or superheat the steam 120 for the reaction.
The effluent stream from the reforming unit 130 is then sent to a shift reactor 150 to obtain additional hydrogen. In the shift reactor 150, the carbon monoxide is reacted with steam to produce more hydrogen. This reaction is usually called the “water gas shift” reaction and the chemical equation for the shift reaction proceeds as follow:CO+H2O=>CO2+H2 The steam reacts with the carbon monoxide from the reforming reaction to produce carbon dioxide and additional hydrogen gas. The reaction energy is exothermic and the heat generated is normally utilized for producing additional steam or heating up other process streams.
The effluent stream from the shift reactor 150 is cooled and sent to a purification unit 160 to separate the hydrogen gas. Pressure swing adsorbers (“PSA”) are often used as the purification unit 160 following steam reformation and shift reaction. Generally, in a PSA process, the gas mixture stream is passed over an adsorbent bed at elevated pressure, whereby the bed selectively adsorbs and holds the impurities. In this respect, the PSA process produces a substantially pure hydrogen product at elevated pressure. The impurities adsorbed on the PSA beds are desorbed at a substantially lower pressure. These desorbed impurities along with unrecovered hydrogen form a low pressure residue gas, which may include carbon dioxide, carbon monoxide, and unreacted methane.
To improve the overall thermal efficiency of the process, it is important to utilize the heat available in the process streams such as process effluents from the reforming unit 130 and the shift reactor 150, and exhaust gas 145 from the furnace 137. In addition to generating or superheating steam, such heat may be used to heat process feed, combustion air for the furnace, and preheat boiler-feed-water. Typically, the amount of steam that may be generated from all available heat exceeds the amount of steam required for the steam reformer 130. The excess steam is normally exported as a byproduct of the hydrogen plant.
As shown in Table 1 below, the residue gas 165 process stream, which may include CO2, CO, CH4, and H2, has a relatively low heating value (e.g., 2419 kcal/Nm3, compared to heating value of 9,090 kcal/Nm3 for natural gas) and is usually at a low pressure of about 1.3 bara. Consequently, use of the residue gas in a hydrogen plant is limited. Typically, the residue gas 165 is recycled to the reformer furnace 137 and used as fuel.
Table 1 below illustrates a typical material balance around the PSA.
TABLE 1ComponentUnitFeed to PSAPure H2Residue GasH2Nm3/h78449698208629CH4Nm3/h63396339CONm3/h44704470CO2Nm3/h1724617246N2Nm3/h38370313FlowNm3/h1068876989036997Heating ValueKcal/Nm3,2419LHVPressureBar a (psig)25.3 (350)24.3 (335)1.3 (5)
One problem encountered during the hydrogen generating process is that the amount of residue gas 165 may limit the flexibility and efficiency of the overall process. One example of the limitation imposed by the residue gas 165 is when the demand of by-product steam is low and suppression of steam production is desired without loss of thermal efficiency of the process. The by-product steam is typically produced by utilizing the heat in the gas exhaust 145 leaving the furnace 137 and the process effluents of the reforming unit 130 and the shift reactor 150. During a period of low steam demand, steam production may be suppressed by redirecting the hot exhaust gas 145 to preheat the natural gas feed 110, the combustion air for the furnace 137, or the fuel stream 140 to the furnace 137. As a result of the preheating, not as much heat is needed in the furnace 137, thereby reducing the requirement of the fuel stream 140. Typically, the residue gas 165 is a major component of the fuel stream 140, while “make-up” fuel, which is input from other sources e.g., fuel supplied from methane feedstock 110, makes up the remaining portion of the fuel stream 140. Because all of the residue gas 165 is fed to the fuel stream 140, a reduction in the fuel stream 140 is generally accomplished by controlling input from the make-up fuel. Thus, the degree of steam suppression depends on the percentage of the make-up fuel stream.
The make-up fuel stream is also used as a means to control the temperature in the furnace. Proper control of furnace temperature typically requires about 10 to 15% percent of heat to be provided by the make up fuel. This percentage of the make up fuel defines the lower limit below which reliable temperature control of the furnace is sacrificed. If the amount of make-up fuel is close to the lower limit, then the amount of make-up fuel may not be adjusted to reduce the furnace fuel firing. Consequently, the residue gas controls the minimum amount of the fuel stream 140 that must be fired to dispose all of the residue gas, thereby limiting the suppression of steam generation.
The following thermal balance example shows the amount of steam made when all PSA residue gas is used, and the net fuel import is at its minimum:
TotalFuel FromMake UP FuelMake upHydrogenFurnacePSAfor Tempfuel as %Exportproduced,FiringResidueControl,of TotalSteamNm3/hGcal/hGas, Gcal/hGcal/hFiringkg/h46000103.149013.1412.752560This thermal balance shows that the make up fuel is only 12.7% of total firing. To reduce the steam generation, the furnace firing needs to be reduced. However, because the make up fuel is close to the lower limit desired for furnace temperature control, the extent of furnace firing reduction is limited due to the fuel balance.
Furnace firing reduction may also be achieved by altering other operating parameters. One such parameter is the amount of steam added to the natural gas at the inlet to the reforming unit 130. Another such parameter is lowering of the reforming unit 130 outlet temperature. In both cases, these changes increase the methane slip at the reforming unit 130 outlet, which, in turn, will increase the amount and heat content of the PSA residue gas 165. However, as shown in the above example, the hydrogen generation process is already limited by the existing amount of residue gas 165, and therefore, cannot accommodate this increase in the residue gas.
There is a need, therefore, for a method to increase the flexibility and efficiency of the hydrogen generation process. There is also a need to reduce the amount of residue gas that must be disposed.